Synthetic orthopedic casting tapes are widely used throughout health care industries. Synthetic orthopedic casting materials are made from a liquid resin-impregnated narrow fabric or casting tape formed of glass fibers and/or synthetic fibers such as polyester, nylon, polyolefins and the like. Orthopedic casts formed from the synthetic casting materials have various advantages over conventional Plaster of Paris casts in that they are lighter, stronger and, due to the nature of the materials used, more breathable.
Water hardenable polyurethane prepolymers disclosed in Yoon, U.S. Pat. No. 4,433,680, constitute a particularly desirable hardenable liquid resin for use in synthetic casting materials. These polyurethane prepolymers, which employ a dimorpholinodiethylether catalyst, have a long shelf life during which the polyurethane polymer remains in a liquid state. When the casting tape is to be used, it is removed from a sealed package and placed in water for a few seconds. It is removed from the water and applied to the patient, usually over a tubular, knitted fabric and a padding. The bandage sets rapidly to a hardened condition in which it is capable of immobilizing a fracture.
Open mesh knit fabrics are widely used in casting tapes because of their inherent ability to stretch and because of their ability to maintain a stable mesh structure when tension is applied along the length and/or width direction of the fabric. Stretch is important because it is highly desirable that the hardened cast conform uniformly to irregular surfaces of a patient's body without causing pressure points. Stretchable narrow fabrics, i.e. tapes, can be applied to the body of a patient without requiring the formation of tucks and/or folds to compensate for irregular surfaces. High porosity, open mesh fabric structures allow the liquid resin carried by the tape to harden at a relatively fast rate and also allow the circulation of air through the hardened cast, thereby improving patient comfort.
Buese et al., U.S. Pat. No. 4,668,563, discloses high modulus casting tapes of improved conformability wherein the casting tape fabric is formed from a combination of high modulus yarns and elastic yarns. The elastic yarns are incorporated into the fabric along the length direction to give the fabric an extensibility of between 40% and 200% in the length direction. Preferably, these fabrics are Raschel Warp Knit fabrics having the elastic yarns forming or distributed within the wale yarns. The elastic yarns are incorporated into the fabric during the knitting process under a predetermined amount of tension so that the finished fabric gathers or bunches to a moderate degree when it is released from the knitting machine. The resulting fabrics are impregnated with a hardenable prepolymer to provide a casting tape of substantially improved conformability that has experienced wide-spread commercial success.
Recently, visibly patterned orthopedic casting tapes as disclosed in U.S. Pat. No. 5,088,484 to Freeman et al. have also achieved substantial commercial success. These casting tapes employ open mesh fibrous tapes bearing coloring agents which are applied to the tape in a visible pattern by various conventional processes including dyeing, sublimation dye printing and ink printing. The visible pattern formed by the coloring agent is stable in the presence of the uncured, hardenable casting resin and also in the presence of the cured, hardened resin.
Patterned casting tapes are preferably provided with colored patterns of substantial optical density and uniformity so that the colored pattern is readily visible on the finished cast. However, highly conformable casting tapes based on highly elastic casting tapes are typically stretched as they are applied to a patient, thus distorting the pattern on the casting tape and also distorting the optical density of the pattern. The distortion of the shape of a visible pattern on an open mesh, highly elastic knit fabric is non-uniform due to the phenomenon of `necking` in which an elastic fabric contracts in its width direction as it is stretched in its length direction. Such necking behavior is particularly apparent in open mesh, highly elastic knit fabrics because of the interlocked nature of a knit fabric structure and because of the relatively low basis weight, i.e, relatively low fabric density, of open mesh knit fabrics. The impact on the printed pattern caused by stretching a highly conformable casting tape is increased by the open mesh structure of the casting tape fabric and by the nature of the fabric itself. In the relaxed state, open mesh knit fabrics based on elastic filaments are generally gathered or thickened. Accordingly, as the fabric is stretched, the optical density of the fabric is decreased both by the increase in porosity of the fabric and by the decreasing thickness of the fabric.
Distortion of visible patterns printed on elastic fabrics as a result of subsequent stretching of the fabric is known as recognized, for example, in U.S. Pat. No. 3,613,679 to Vijou. This patent proposes the application of a visible pattern to an elastic bandage of the type employed to support a sprained joint or the like by wrapping the bandage around an injured joint under tension. An undistorted pattern is applied to the elastic bandage by printing while the bandage is stretched. When the bandage is subsequently relaxed, the pattern is distorted. When the bandage is applied to a patient, a predetermined amount of tension can be applied by the bandage by stretching the bandage sufficiently to return the pattern to its undistorted state. Although in theory the variations in the distortion of the pattern should uniformly indicate a desired amount of tension on the elastic bandage, U.S. Pat. No. 4,437,408 to Arkans indicates that because elastic webs have different stretch characteristics between different lots or types of webs, changes in the geometric form are not necessarily uniform. According to this patent, uniformity of pattern distortion in elastic bandages can be achieved by printing a geometric form onto the elastic bandage while maintaining the bandage under a predetermined measured amount of tension rather than a predetermined amount of stretch.
Despite the expectation of the prior art that printing of a geometric pattern onto a stretched elastic tape under controlled tension should provide an elastic tape exhibiting the same, undistorted printed pattern when the tape is subsequently stretched the same amount as it was stretched during printing, it has been found that open mesh, knit orthopedic casting tapes based on elastic filaments do not uniformly follow the expected behavior. Thus when printed under tension using conventional printing processes and thereafter coated with liquid resin using conventional coating equipment, these tapes in many cases do not exhibit an undistorted pattern when they are subsequently stretched in about the same amount as during printing. There are substantial differences between open mesh casting tapes and elastic bandages including substantial differences in the power of the tapes. The low power of casting tapes renders these tapes more susceptible to stretching upon variations in tension. In addition, thickness changes and necking are more pronounced in casting tapes upon stretching.